1. Field of the Invention
The invention relates to an apparatus for scattered light inspection of optical elements, in particular of lens surfaces, having a light-generating unit for generating light that is irradiated onto the optical element respectively to be inspected, and a detector for detecting scattered light that is emitted by the optical element during irradiation.
2. Description of the Related Art
Such apparatuses are variously known and are chiefly used to detect the quality of optical elements such as, for example, lenses, by means of scattered light inspection. This is particularly important for optical elements that are used in high-resolution optical systems. Compared with traditional, visual inspection, the use of scattered light inspection apparatuses enjoys the advantage of a higher level of objectivity owing to their angularly resolved quantitative measurement, and of a better possibility of documentation. Moreover, the risk of damaging the optical element to be inspected is less. One application is optical elements of exposure machines in microlithography for the purpose of semiconductor wafer exposure. Surface roughnesses on surfaces of lenses used there, for example, in a projection objective, which can be caused by contaminants or surface damage have given rise in the exposure mode to parasitic scattered light that falls outside the structures to be exposed onto a wafer and thereby reduces the contrast of the imaging. This has direct effects on the minimum achievable critical dimension of wafer structures, and on its uniformity.
The closer the scattering surface is to the field, the more strongly the scattered light is concentrated onto the near zone about the structure to be exposed. Such so-called short range scattered light has the greatest influence on the uniformity of the minimum achievable critical dimension, and is therefore particularly disturbing during the lithography process. Scattered light inspection is therefore useful, in particular, for the surface of a first lens, closest to the field, in a microlithography projection exposure machine in order to determine whether and, if appropriate, which surface roughnesses and/or surface contaminants this lens has. It is also frequently desirable to perform scattered light inspection of the lens on the output side, closest to the wafer, of a projection objective of the exposure machine, since this lens is exposed to instances of degassing, for example of photoresist layers of a wafer to be exposed.
Surface contaminants are observed on virtually all customary DUV and VUV projection exposure systems. Various contaminant morphologies are to be observed, depending on composition and origin, such as dendrites, spherical shapes, flat sharp-edged crystals, etc. In this case, there also seem to be relationships between the microscopic appearance of contaminants and their ability to be removed.
Scattered light inspection apparatuses are conventionally stationary systems into which the optical element respectively to be inspected is to be introduced. In this case, these systems typically comprise a number of modules that are to be handled individually and each have a dedicated housing, for example a laser unit as a first, light-generating module, and a scattered light sensor as a further, autonomous module. In the case of the abovementioned microlithography projection objective lens, this means that such a stationary apparatus can be used to inspect the lens only before it is firstly installed in the microlithography machine, and thus before its use in the exposure mode, or that the lens must be removed for inspection. Firstly, this necessitates outlay on staff, and secondly lengthy times, typically of several weeks, on downtime for the system, and this renders regular inspection completely uneconomical.
Such stationary scattered light inspection systems are known in a great variety of designs—see, for example, patents U.S. Pat. No. 3,652,863 and U.S. Pat. No. 3,814,946 and the laid-open patent applications DE 44 23 802 A1, DE 197 39 679 A1 and DE 198 27 183 A1.